Antennas are typically of two types, namely symmetrical or balanced, and asymmetrical or unbalanced. FIG. 1 depicts a typical unbalanced antenna 100. The antenna 100 includes monopole 101 and ground 102. The input or feed structure is also unbalanced, and may be a coaxial cable 104 with ground shielding 103 or a micro-stripline (not shown). The unbalanced antenna has a single element for the total energy of the signal, which alternates + (positive) and − (negative). Note that the ground plane functions as part of the antenna, and thus strongly affects the performance of the antenna. The antenna can be detuned if the size of the ground place is between 0.25 and 2 wavelengths of the antenna resonant frequency. Other elements connected to the ground plane can also detune the antenna. Since antenna resonant frequency and performance depends on the shape of the device, each antenna needs to be customized, leading to higher design and production costs. However, since there are multiple radiating elements, the antenna 100 is useful for multi-band applications, e.g. mobile phones. However, if the RF module that provides the signal to the antenna 100 is balanced, an additional balun type antenna is required, which introduces additional losses and decreases the antenna's radiation performance. Examples of antenna 100 are monopole, patch, and PIFA (planar inverted-F antenna).
FIG. 2 depicts a typical balanced antenna 200. The antenna 200 includes loop 201 with ground 204. The input or feed structure is also balanced, and comprises separate + input 202 and − input 203 for each part of the alternating signal. The feed structure may comprise a coplanar microstrip line or two-wire transmission line. Note that with this arrangement, the ground plane is essentially independent from the antenna and has little effect on the performance of the antenna. Thus, the antenna resonant frequency and performance depends on the shape of the device, and a single antenna can work with a variety of ground plane geometries. However, since this arrangement has a symmetric geometry, the size of the antenna is double that of an equivalent unbalanced antenna. This antenna has a single radiating element and can be configured to operate in wide-band single resonance applications, such a magnetic resonance imaging (MRI) device and other inductive coupling applications.
In designing electronic circuits, e.g. mixers or amplifiers, balun antennas are used to link a symmetrical or balanced circuit with an asymmetrical or unbalanced circuit. Thus, a balun can be used to change an unbalanced signal to a balanced signal in order to drive a balanced antenna element, or vice versa. FIG. 3 depicts a typical Marchand type balun antenna 300. The Marchand balun has an unbalanced input 303 and a balanced output 301. The input goes to two coupled line sections 304, 305, the lengths of which are λ/4 (a quarter wavelength) of the input signal. The portions of the line sections that are connected to the outputs are shorted to ground. The portions of the line sections that are connected to the input are connected to an open circuit (OC). The Marchand balun operates through the coupling that occurs between the lines. The balun offers good amplitude balance and phase difference with a relatively wide operating bandwidth.
Note that in the balun of FIG. 3, the operating bandwidth is mainly controlled by the coupling strength of the two coupled-line sections. There are two types of coupled-lines, namely edge coupled and coplanar coupled. FIGS. 4A and 4B depict examples of edge coupled lines 401, and coplanar coupled lines 402, respectively. In FIG. 4A, the signal line 403 couples with line 406. The couple lines are separated from a ground place 404 by dielectric material 405. This coupling is referred to an edge coupling. With this arrangement, manufacturing capability limits the coupling strength between a pair microstrips. In FIG. 4B, the signal line 403 couples with line 406. The couple lines are separated by dielectric material 405. Ground plane 404 are adjacent to the couple lines. This arrangement is referred to as a coplanar waveguide configuration or broadside configuration, where one coplanar waveguide (e.g. 403) is on the top of the dielectric 405 and another coplanar waveguide (e.g. 406) is on the bottom of the dielectric. Strong coupling can be achieved by a pair lines in this arrangement.
There are two types of coplanar coupling, namely symmetrical and asymmetrical. FIGS. 5A and 5B depict examples of symmetrical 501 and asymmetrical 502 coplanar coupled lines, respectively. In FIG. 5A, the signal line 503 is coplanar with line 504 and separated by dielectric layer 508. The ground planes 505 and 506 are also coplanar and separated by dielectric layer 508. In FIG. 5B, the signal line 503 is coplanar with line 504 and separated by dielectric layer 508. However, the ground planes 505 and 507 are not arranged in the same manner as the signal lines. This arrangement is referred to as asymmetric coplanar striplines (ACPS), and can be used to reduce the space for grounding, while still achieving strong coupling. The ACPS striplines will also have a wide bandwidth as the symmetrical coplanar striplines of FIG. 5A. A Marchand balun based on ACPS coupling has a small size and a wide operating bandwidth.
Inhomogeneous media can cause a large difference between the even-mode and odd-mode velocities. A large difference degrades the performance of the balun. An arrangement that has a nonuniform ACPS that is covered with a dielectric can be used to overcome this problem. FIGS. 6A and 6B depict different views of a nonuniform ACPS coupler 600. In this arrangement, the ground place is formed into an irregular shape. This arrangement improves performance of the bandwidth, because it reduces the difference in the even and odd mode velocities through the waveguides.